1. Field of the invention
The invention relates to a system for detecting friction between an object, that is being printed in a printer, and a print head reciprocating relative to the object.
2. Description of the Related Art
When a print head of a scanning-type printer moves relative to an object, e.g. relative to a substrate on which the object is printed in a layerwise way, a collision between the print head and the object may cause serious damage to the object and/or the print head. In order to limit such damage, it is known to automatically detect collisions between the print head and the object and to cause an emergency stop of the printer as soon as a collision is detected.
U.S. Pat. No. 8,579,405 B2 discloses an ink jet printer which is also capable of detecting a situation in which the print head does not experience a hard collision with the print substrate but just gently touches the print medium with its nozzle face. This is achieved by counting the number of nozzles of the print head which show malfunction due to direct contact with the print medium.
U.S. Pat. No. 6,055,391 discloses a printer in which acoustic microphone based systems are used for detecting characteristic vibration signatures that are indicative of machine breakage, machine wear, and the like.
In the art of relief printing or 3D-printing, an elevated, three-dimensional object is printed by superposing several layers of curable ink. Consequently, the height of the top surface of the object on which a new layer of ink is to be deposited increases with every layer that is being added. On the other hand, in order to achieve a high print quality, the distance between the print head and the surface of the printed object should be kept as small as possible. It is therefore necessary to control the height of the print head in accordance with the gradual build-up of the printed object. Thus there exists a risk of an event where the print head comes into frictional contact with the printed object, for example due to errors or tolerances in the height control of the print head, tolerances in the height of a substrate on which the object is being printed and which is supported and fixed by means of jigs, adhesive tape or the like, or else due to a false estimate for the exact thickness of the layers of ink that are deposited in each print pass.
When an event of friction between the print head and the printed object occurs, this may not only compromise the quality of the printed object but may also cause damage to the print head.
It is therefore an object of the invention to provide a system for reliably detecting friction between the printed object and the print head.